THE CHEMICAL-COMPOSITION OF XENOTIME FROM THE LIMOUSIN REGION, MASSIF-CENTRAL, FRANCE

Fifty-one specimens of xenotime from the Limousin region, in the Massi f Central of France, have been analysed with an electron microprobe; t wenty-nine come from stream sediments derived from leptynites (metagra nitic rocks poor in ferromagnesian phases), seven come from leptynites , and fifteen come from postmetamorphic granites. In these, xenotime h as been found only in some peraluminous granites, the biotite of which has a restricted chemical composition in Al2O3 - FeO, Al2O3 - MgO and MgO - FeO binary plots. All the specimens of xenotime analysed show i mportant replacement of Y by the heavy rare-earth elements (HREE), mai nly Dy, Er, and Yb. The level of incorporation of the HREE remains con stant (between 18 and 19% by weight of the oxides). In contrast, the l ight REE amount to less than 1%. Erbium shows the lowest variations in concentrations, whereas Nd, Sm, Gd and Ho vary widely. The heaviest R EE (Yb, Lu) are concentrated in the core of crystals. Substitution of the actinides for Y and of Si for P is very limited. The U contents ar e invariably higher than the Th contents; the greatest differences bet ween levels of U and Th are observed in xenotime from granites. Xenoti me from leptynites and that from postmetamorphic granites are very sim ilar in chemical composition; the latter is richer in Nd, Sm, and U. I n the granites studied, xenotime hosts only a small proportion of the whole-rock yttrium content. This element thus must also be located in other minerals, especially zircon, garnet, monazite and titanite. Xeno time from leptynites and postmetamorphic granites of the Limousin diff ers from that of other origin (granitic pegmatites, alkali biotite-bea ring granites, alpine clefts, chlorite-bearing alteration assemblages) in its concentrations of either specific REE or U and Th.